1. Field of the Invention
This invention relates to measuring gamma radiation from a subsurface formation. In particular, the measuring is performed within a borehole.
2. Description of the Related Art
Geologic formations below the surface of the earth may contain reservoirs of oil and gas. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Generally, the oil and gas are retrieved by drilling boreholes into the subsurface of the earth. The boreholes also provide access to take measurements of the geologic formations.
The geologic formations may include formation layers. In a quest for oil and gas, it is important to know about the location and composition of the formation layers. In particular, it is important to know about the formation layers with a high degree of accuracy so that drilling resources are not wasted.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. In one embodiment, a “logging instrument” is lowered on the end of a wireline into the borehole. The logging instrument sends data via the wireline to the surface for recording.
Measuring naturally occurring gamma radiation (or gamma rays) is one way to determine characteristics of the formation layers. A gamma radiation detector may be used as a component of the logging instrument to measure the gamma radiation. In typical embodiments, a scintillator material and a photomultiplier are used for gamma radiation detection. A gamma ray entering the gamma radiation detector will cause an output of an electrical signal. The output may be referred to as a “count.” The magnitude of the electrical signal is generally related to the energy of the gamma ray detected.
Gamma rays may be emitted from the formation layers and from borehole materials such as mud. In particular, the mud may contain gamma radiation emitters such as clay and potassium. As the gamma radiation detector moves along the borehole, gamma rays from all sources may enter the gamma radiation detector and be detected. In addition, the gamma radiation detector may detect an energy level of each gamma ray entering the detector. The gamma ray energy levels may be used to create gamma ray energy spectra. The gamma ray energy spectra reflect types and quantities of elements (referred to as elemental yield) in the formation layers and the borehole materials.
The borehole generally contains a drilling mud or in the case of a producing well some mixture of hydrocarbons and water. Unfortunately, an exact elemental make up of the fluids in the borehole, and in particular the drilling mud, is neither constant nor well known. The drilling mud contents are determined to a large degree by a mud company. Generally, ingredients of the drilling mud are trade secrets that are not available to a well owner or a service company logging a well. In addition as the well is drilled, the drilling mud is contaminated by drill solids. The drilling solids are a mixture of rocks that have been drilled through. As the drilling mud is circulated and the drill solids are partially removed at the surface, mineralogy and lithology and, hence, elemental composition is not sufficiently known to completely correct the gamma ray energy spectra or computed elemental yields. Further, even if an elemental composition of the borehole materials is known, the cross-sectional area of the borehole is not constant. Hence, correction to the gamma ray energy spectra and, therefore, the computed elemental yields also depends on the borehole caliper and a position of the logging instrument in the borehole.
Accuracy of determinations of the mineralogy and lithology of the formation layers depends on the accuracy of data used to determine the type and quantity of elements in the formation layers. In general for oil and gas exploration purposes, interest lies in the type and quantity of the elements in the formation layers. In that gamma radiation may be emitted from the borehole materials in addition to the formation layers, it is critical to correctly partition elemental yields between the borehole materials and the formation layers.
The drilling mud in the borehole can also influence gamma radiation detection by absorption of the gamma radiation emitted by the formation layers. Generally, the gamma radiation absorption is accounted for by correction procedures, charts, mud density measurements, and borehole diameter (caliper) measurements for centralized and decentralized logging instruments.
An instrument suitable for measuring gamma radiation is a SPECTRALOG logging instrument, commercially available from Baker Hughes Incorporated of Houston, Tex. The SPECTRALOG logging instrument provides elemental concentration data for uranium, thorium, and potassium.
Another method of measuring characteristics of the formation layers using measurements of gamma radiation is referred to as “pulsed neutron spectroscopy.” The pulsed neutron spectroscopy method uses neutron activation of nuclei in elements of the formation layers. A FLEX logging instrument, commercially available from Baker Hughes Incorporated, uses pulsed neutron spectroscopy. In general, a pulse of neutrons is used to activate the nuclei. The neutrons interact with the nuclei, which may emit characteristic gamma rays through one of three processes, inelastic neutron scattering, fast-neutron reactions or neutron capture. The gamma rays from the inelastic and fast-neutron interactions occur during, or very soon after, the pulse of neutrons. The gamma rays resulting from the capture events occur later. As above, the gamma rays are detected with the gamma radiation detectors. The gamma rays resulting from fast interactions are used to create inelastic gamma ray energy spectra while the gamma rays occurring later create capture gamma ray energy spectra. Data from the two types of gamma ray energy spectra are used to deduce the elemental yields. The data are corrected to account for the inelastic and fast-neutron interactions and the neutron capture in the borehole materials. For the reasons discussed above concerning variations and unknowns of the drilling mud, accuracy of corrections applied to the data may not be known.
What are needed are an apparatus and a method for making measurements of gamma radiation of a subsurface formation from a borehole. In particular, the apparatus and method are needed to compensate for gamma radiation emitted from the borehole materials.